Biology of the N2-fixing soil bacterium, Sinorhizobium meliloti

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The soil bacterium, Sinorhizobium meliloti, forms N2-fixing nodules on the roots of alfalfa. N2-fixation is of huge significance to agriculture as it by-passes the economic and environmental costs associated with nitrogen fertilizer. In addition to nitrogen (N), the growth of plants and microorganisms is often limited by the amount of available phosphorus (P) (primarily present as inorganic phosphate (Pi)). In this proposal we outline studies that focused on how bacteria transport Pi into their cells and how bacteria sense Pi and respond and survive when Pi is limiting growth. We will study a Pi transport system called Pap-Pit that is present in many bacteria. How this system functions, and particularly what is the role of the Pap protein is not known. We propose that Pap modulates or controls the activity of the Pit transport protein and we perform experiments to address this hypothesis. Another protein, PhoU, has some structural similarity to Pap, and we will investigate whether PhoU controls a different Pi transporter called PstSCAB and how this system is involved in sensing the cellular "Pi-status". The S. meliloti genome is unusual, as in addition to a chromosome (3.7 Mb), it has two megaplasmids called pSymA (1.4 Mb) and pSymB (1.7 Mb). We will investigate the role of the megaplasmids in Pi utilization and how loss of these megaplasmids (40% of the genome) affects the integration of carbon and Pi metabolism in these cells.